mysql-convert

1.convert(value,type);

将value转换为指定的类型b；

sql语句要以分号结尾，通过向上键选择想要直接使用或修改较少的sql语句。

2.cast(value as type);

3.ifnull(a,b);

select ifnull(a,b);如果a不为null,则选择a,反之则选择b；

4.limit a,b

从第a个开始，找b个值

5.top number/percent

找最前边的number个值

6. @变量名，:=进行赋值，且需要在from字句中进行初始化
SELECT Score, CONVERT (

    CASE 
    WHEN @preValue = Score THEN @preRank
    WHEN (@preValue := Score) >= 0 THEN @preRank := @preRank + 1
    END,

    UNSIGNED) AS Rank
FROM Scores, (SELECT @preValue := NULL) p, (SELECT @preRank := 0) v
ORDER BY Score DESC;

7.select Score,(select count(distinct Score )from Scores as s2 where s2.Score>=s1.Score) Rank
  from Scores as s1
  order by Score DESC;

8.排名一定要使用distinct，相同的值属于同一排序值。

9.sql执行顺序

标准的 SQL 的解析顺序为:

(1) FROM 子句 组装来自不同数据源的数据

(2) WHERE 子句 基于指定的条件对记录进行筛选

(3) GROUP BY 子句 将数据划分为多个分组

(4) 使用聚合函数进行计算

(5) 使用HAVING子句筛选分组

(6) 计算所有的表达式

(7) 使用ORDER BY对结果集进行排序

10.set @cnt := 0, @lnum := -101;
select distinct t.Num ConsecutiveNums from (select Num, (case when Num <> @lnum then @cnt := 1 else @cnt := @cnt+1  end) as  cnt,  @lnum := Num from Logs)t where t.cnt >=3;

11.SELECT D1.Name Department, E1.Name Employee,  E1.Salary  
FROM Employee E1, Employee E2, Department D1  
WHERE E1.DepartmentID = E2.DepartmentID  
AND E2.Salary >= E1.Salary   
AND E1.DepartmentID = D1.ID        
GROUP BY E1.Name  
HAVING COUNT(DISTINCT E2.Salary) <= 3  
ORDER BY D1.Name, E1.Salary DESC;

12.delete from person where id not in (
select id 
from (select min(id) as  id from person group by email) p)